With pneumatic tires for heavy loads, uneven wear (such as step wearing) occurs in the shoulder region of the tread under conditions of use such as when the steering shaft is mainly turned while driving continuously at high speeds for a long period of time. Conventional pneumatic tires have sought to solve this problem by providing a circumferential fine groove that extends in the circumferential direction of the tire formed in the shoulder region of the tread for the purpose of preventing uneven wearing of the shoulder region of the tread. A rib that extends in the tire circumferential direction is formed on the outer side in the tire width direction by the circumferential fine groove. Furthermore, when the tire is in contact with the ground, this rib minimizes uneven wear in the shoulder region of the tread by proactively wearing as an abrasion sacrifice rib (for example, refer to Japanese Examined Patent No. H06-004364 and Japanese Patent No. 3902240).
Specifically, the pneumatic tire for heavy load shown in Japanese Examined Patent No. H06-004364 has a shoulder rib that is divided into an outer rib (sacrifice rib) and an inner rib by a single auxiliary groove. The auxiliary groove is set at an interval corresponding to between 25 and 40% of the width of the shoulder rib as measured from the end edge of the tread. Furthermore, the auxiliary groove is set to be angled toward the outside at an angle of 20 degrees or less with regards to the normal line of the tread surface.
Furthermore, the pneumatic tire for heavy load according to Japanese Patent No. 3902240 has a sacrifice rib surface that is offset and recessed to the inner side in the tire radial direction from the profile line in the tire width direction of the shoulder rib, and the amount of offset is between 0.5 and 2.0 mm for a new tire.
However, with the pneumatic tire for heavy load shown in Japanese Examined Patent No. H06-004364, the ratio of the width of the sacrifice rib to the width of the inner rib of a new tire will gradually increase when the new tire becomes worn and will exceed the specified level of between 25 and 40%, based on the location of the auxiliary rib on the shoulder rib and the angle thereof. Therefore, there is a concern that rigidity of the shoulder rib will drop when the new tire becomes worn, resulting in deterioration of uneven wear resistance.
Furthermore, with the pneumatic tire for heavy load shown in Japanese Patent No. 3902240, the surface of the sacrifice rib is formed to be offset and recessed to the inner side in the tire radial direction from the profile line in the tire width direction of the shoulder rib. Since a drop in road contact pressure at the outside edge part in the tire width direction of the shoulder rib is thus induced at the early stages of wearing, there is a concern that uneven wear will be accelerated.
In light of the foregoing, an object of the present invention is to provide a pneumatic tire for heavy load that can increase resistance to uneven wear.